1. Field of the Invention
The invention relates to a light emitting diode cluster unit comprising an array of light emitting diodes and suitable for use in display signs such as those used on highways to display information to motorists.
2. Prior Art
Display signs used for advertising or for displaying direction or other information to motorists have an important feature in common. They should be eye-catching and their information should be easy to absorb. This is especially true of signs to be located on the highway where the motorist may be moving fast and may need to absorb the information quickly.
Such display signs have traditionally used fluorescent or incandescent lamps which are expensive in electricity and may need frequent maintenance and repair. Each lamp usually forms an appreciable part of any letter in a display of information, and so, if any one lamp becomes non-functional it has an appreciable effect on decipherability and as more lamps become non-functional it may be considerably more difficult to decipher the letter.
U.S. Pat. No. 4,727,289, issued 23rd Feb., 1988 to Akio and assigned to Stanley Electric Co. Ltd., discloses a lamp comprising a plurality of high-emitting diodes LEDs on a printed circuit board and arranged inside a glass bulb. Current is supplied to the LED through a series resistor having an annular shape and fitted around the stem of the lamp. Lamps according to that patent may be used for displays while providing advantages such as:
1. the provision of a very large number of discrete light sources (LEDs). If only a few of the LEDs fail, there will be little visual deterioration to the display,
2. by suitable selection of the LEDs, the colour of the display or parts of the display is easily adjustable without the use of tinted glass or other filters, and
3. a considerable saving in energy cost.
However, it might be desirable to reduce the expense of manufacture of such LED arrays within conventional glass bulbs. Moreover, when used for displays, the glass bulb itself may produce some glare and reflection of, example, sunlight, thus tending to confuse the display. Also the reliability of the connector socket may not be high due to the fact that the contacts are not environmentally sealed and are therefore susceptable to corrosion. Corrosion in the socket may make it difficult to replace the bulb especially in harsh environments such as highways.